Annealed, cold-finished steels



ANNEALED, COLD-FINISHED STEELS Elliot S. Nachtman, Park Forest, 11]., assignor to La Salle Steel Company, Hammond, Ind., a corporation of Delaware No Drawing. Application October 22, 1956 Serial No. 617,271

Claims. (Cl. 148-12) This invention relates to the improvement in mechanical and physical properties of steel by a process capable of being used in the cold finishing of steel, as for example, in the cold finishing of steel bars, rods, wires, tubing and the like steel products.

'It is an object of this invention to produce and to provide a method for producing cold finished steel products having new and improved physical and mechanical properties.

More specifically, it is an object of this invention to provide a method applicable to the metallurgical processing of hot rolled steel or steels of the type which strain harden and which harden by some mode of precipitation during working at elevated temperatures to improve the physical and mechanical properties of the steel, and it is a related object to produce new and improved cold finished steel products by the use of such method in the process of cold finishing steel.

This application is an improvement of the copending applications Serial No. 518,411, Serial No. 518,412, Serial No. 518,413 and Serial No. 518,414, filed by Nachtman' and Moore on June 27, 1955, now Patents No. 2,767,837,.

2,881,108 Ce Patented Apr. 7, 1958 properties and the physical and mechanical characteristics of the steel, but that the steel can be still further varied, and in some instances improved in its physical and mechanical properties, such for example in ductility, residual stress, load required for advancement of the steel through the die, and in such strength characteristics as tensile strength, fiexure strength, impact strength and the like.

As used herein, the term physical and mechanical properties is meant to include tensile strength, yield strength, machineability, hardness, ductility, elongation,

' residual stress, warpage factor and the like. It has been found, for example, as described in the aforementioned copending applications, that the machineability characteristics of the steels and such properties as tensile strength, yield strength, proportional limit, impact strength and hardness are most beneficially effected when, as de- No. 2,767,835, No. 2,767,836, and No. 2,767,838, respec- Q tively.

In the aforementioned copending applications, reference is made to the improvement in physical and mechanical properties of steel by means which include the step of advancing the steel through a die to effect reduction in cross-sectional area while the steel is at an elevated temperature within the range of 200 F. to the lower critical temperature for the steel composition, such for example as when the steel is at a temperature within the range of 200 F. to 1200-1300" F. In accordance with the teaching of the aforementioned copending applications, the temperature employed in the elevated temperature reduction step has considerable influence on the trend of the improvements securedin physical and mechanical properties of the steel. By controlling temperature in combination with chemistry of the steel and the amount of reduction that is taken in the steel, it becomes possible selectively to adjust physical and mechanical properties developed in the steel over a fairly wide range while enhancing the uniformity of physical and mechanical properties in the steel product from heat to heat.

This invention embodies the concepts set forth in the aforementioned copending applications coupled with the additional step of annealing the steel prior to advancement of the steel through a die to effect reduction in cross-sectional area while the steel is at an elevated temperature within the range described, that is, while the annealed steel is at an elevated temperature within the range of 200 F. to the lower critical temperature of the steel composition, or about 1200-1300 F. It has been found that, by the combination which includes annealing of the steel prior to advancement of the steel through the die to efiect reduction in cross-sectional area,-

scribed in the copending applications Serial No. 518,414 and Serial No. 518,413, the steel, in the elevated temperature reduction step, is advanced through the die to, effect reduction in cross-sectional area while the steel is at a temperature within the range of 450-850 F. Even within this particular temperature range, it has been found that tensile strength and yield strength and proportional limits will be maximized when the elevated temperature reduction step is taken while the steel is at a temperature within the range of 450-600 F., while more noticeable improvements in the plastic properties of the steel, such as elongation, reduction of area and impact strength, will be maximized when the elevated temperature reduction step is taken while the steel is at a temperature within the range of 600850 F. These same concepts carry over into the process of this inven-,,

tion wherein the described elevated temperature reduction step is employed in combination with the step of annealing the steel prior to elevated temperature reduction to provide still further modifications and improvements in the characteristics and in the properties of the steel products that are formed.

Aside from and in addition to the. described improve.-

ments in physical and mechanical properties of the steel,. the elevated temperature reduction step provides means. by which a desirable control can be maintained over thestress characteristics that are developed in the steel prod ucts. When, for example, steel is advanced through the die to effect reduction in cross-sectional area while the steel is at a temperature in excess of 650 F., and preferably at a temperature above 850 F. but below the lower, critical temperature of the steel composition, the magnitude of the residual stresses developed in the steel can be materially reduced and the type of residual stresses in the steel can be cont-rolled to produce steel products:

having greatly improved warpage characteristics and a much more desirable distribution ofstresses throughout the cross-section of the steel.

The marked reduction in warpage values that are obi-- tained when the steel is reduced in an elevated temperature reduction step permits the production of steel prod-- ucts having improved physical and mechanical properties with residual stress values-as low or lower than values which have heretofore been capable of being developed from the use of heat treatments or stress-relieving steps following drawing or the like reduction process. It has been possible, in accordance with the practice of this in vention, to produce steel products having compressive stresses, as distinguished from tensile stresses, in the sur face portions of the steel When, as described in the aforementioned copending application Serial No. 518,412, the steel is advanced through a die to efi'ect reduction in cross-sectional area while the steel is at a temperature above 800 F. but below thelowereritical temperature;

for the steel composition and when, almost immediately after reduction, the steel is rapidly cooled to ambient temperature, as for example by quenching in water or oil. The development of high compressive stresses in the surface portions of the steel is desirable to increase the torsional fatigue value of the steel at any particular strength level and to reduce waste, as by cracking, in the fabrication of parts from the steels that are produced.

When, in accordance with the practice of this invention, the elevated temperature reduction step described in the aforementioned copending applications is combined with the step of annealing the steel in advance of the elevated temperature reduction step, still further and important characteristics are developed in the steel. One of the important improvements incapable of being demonstrated by comparison of properties of the steels resides in the ability to give greater uniformity of properties from bar to bar between heats as distinguished from the wide variations of properties, inherent in hot rolled steel bars. Another important advantage which is made available by the combination of steps described over cold drawing or elevated temperature reduction without first annealing resides in the marked increase in the ductility of the steel having the annealed structure, and the marked improvement in machinability characteristics. These improvements are made available while maintaining the high level of strength properties developed by reduction at elevated temperature.

As in the aforementioned copending applications, the trend of the improvements in physical and mechanical properties and the extent of improvement in any one of the properties is influenced to some degree by the temperature of the steel being reduced, the chemistry of the steel, and the amount of reduction that is taken. The rate of cooling of the steel after reduction has very little influence on the properties and characteristics of the steel products except as they influence the formation of compressive stress values, as defined in the aforementioned copending applications.

Steels that respond in the manner described to the combination of steps comprising annealing and reduction at elevated temperature, can be classified quite generally as hot rolled steels or steels of the type which are characterized by the ability to strain harden and also harden by some mode of precipitation or other rearrangement when worked at an elevated temperature in the range employed in the elevated temperature reduction step. Typical of the classing of steels are the hot rolled, non-austenitic steels which have a pearlitic structure in a matrix of free ferrite and are relatively easy to draw. These can be distinguished over the high alloy steels or the hard-todraw, high-speed or carbon tool steels, such as are described in the Kronwall Patent No. 2,440,866. It is not known what the change is that takes place in the steels to bring about the differences in the development of properties because of the step of annealing in advance of reduction at elevated temperature, but it is known that the properties developed in the steel products differ in many ways over steels which are drawn or otherwise reduced at elevated temperatures without having previously annealed the steel.

For effecting reduction at elevated temperature (ETD), use may be made of a conventional drawing or extrusion operation, wherein the steel is advanced through a conventional draw die or extrusion die for effecting a reduction in cross-sectional area. While the process of rolling is not equivalent to the process of drawing or extrusion in a cold finishing operation, it has been found that many of the characteristics and properties capable of being developed in steel by drawing or extrusion at elevated temperature are also capable of development in steels which are reduced in area by a rolling operation while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition after 4 having first annealed or produced an annealed structure in the steel.

As used herein, the term -annealing" is meant to embody the application of the term as ordinarily employed in the steel trade. It includes the heat treatment of steel to annealing temperature for the steel composition, followed by slow cooling to ambient temperature. In the practice of this invention, the steel may be cooled from annealing temperature to the desired elevated temperature for advancement of the steel through a die to effect reduction in cross-sectional area. In the alternative, the steel may be cooled down from annealing temperature to a temperature below that at which the steel is to be advanced through the die in the elevated temperature draw ing step (ETD). In the latter system, the annealed steel would be reheated to the desired elevated temperature for advancement of the steel through the die in the elevated temperature reduction step. In any event, an annealed structure is provided in the worked product or in the product worked where such working strain hardens the steel. Sub-critical annealing may be used wherein the steel is heated to just below its critical temperature, or the steel may be given a fullannealing treatment wherein the steel is heated to above its critical temperaure and slowly cooled.

The concepts of this invention will hereinafter be illustrated by reference to the processing of three steels which may be taken as representative of the steels which may be employed. These representative steels will hereinafter be referred to as C-1144, C-1080 and 4140, having the following ladle analyses in which the major ingredients, other than iron, are set forth:

The procedures for processing the steels and the conditions for testing in the development of the data set forth in the following tables will now briefly be described.

Pr0cedure.The hot rolled steel bars, as received, were descaled by pickling in sulphuric acid and limed to prevent rusting.

The hot rolled, pickled, and limed bar stock processed in accordance with the practice of this invention, was heated to annealing temperature in a suitable heat-treating furnace, such as a Hevi Duty electric furnace. The C-1l44 steel was annealed at a temperature of 1450 F. The C-1080 steel was annealed at a temperature of 1450 F., and the 4140 steel was annealed at a temperature of 1500 F. Annealing followed the usual procedures employed in the steel trade. The annealed steel was then slowly cooled, as previously described, to room temperature.

The annealed bar stock was reheated to the elevated temperature for drawing in a suitable reheating furnace such as a gas-fired furnace.

The steel bars were lubricated in advance of drawing. The reduction steps in the case of 1144 steel were taken by advancing the steel through a conventional draw die. The reduction steps in the case of C-l080 and 4140 steels were taken by advancement of the steel through a draw die of the type described in the Kyle et al. application Serial No. 484,726, filed January 28, 1955.

Definitions.-Percent reduction is meant to relate to the true reduction as represented by the formula X =pereent reduction wherein D is the original hot rolled diameter of the steel and D is the final diameter of the steel.

"Proportional limits" is intended to correspond to the point on the stressstrain curve where the greatest stress that the material is capable of sustaining without deviation from the law of proportionality of stress to strain occurs (Hookes law). This point is of particular importance in steel and, in practically every instance, it is measurably increased to exceptionally high values when steels, such as steels of the non-austenitic type, are processed as by drawing or extrusion while the steel is at an elevated temperature within the range described.

Warpage factor is directly related to residual stress. The warpage value is an indication of the concentration and character of the longitudinal stresses present in steel. The residual stress is obtained by means of a warpage test wherein the length of the test piece is determinedas being five times the diameter plus 2 inches. The test pieces are slotted through a diameter for a distance five times the diameter. The length of the slot is recorded and the maximum diameter perpendicular to the slot is also recorded. The differences between the diameter before slotting and after slotting represent the flare caused by the presence of residual stresses. The flare is considered positive, indicating a preponderance of tensile stresses, if the bar expands on slotting. The flare is considered negative, indicating a preponderance of compressive stresses, if the ends move towards the out which is made through the diameter. The warpage values determined for evaluation are calculated on the following equation:

(LS), x100 Warpage factor= where D =the original diameter of the bar before cutting the slot D =the diameter difierential before and after cutting the slot (flare) L =length of slot her (DPN), was measured on a Gries reflex testing machine employing a 136 Pyramid Diamond at a kg. load.

The term elevated temperature drawing (ETD) is meant to define the taking of a reduction in the cross section of the steel by advancement of the steel through a die while at a temperature within the range of 200 F. to the lower critical temperature for the steel composition (1l00-1200 F.). In the data which will hereinafter be set forth, the steel as received was of the following dimension:

C-1144 /8 round 0-1080 round 4140 5 round The data set forth in the following tables relates to the values of the hot rolled and annealed steels and annealed steels drawnto effect an equivalent reduction at ambient temperature (cold drawn), and the same annealed steels drawn at an elevated temperature (Ann.+ETD). The last represents the improved practice of this invention. In the development of the data, steels of the same chemistry were employed for the various tests. The amount of reduction for the same steels was held as nearly comparable, one with the other, as possible, and the reduction at elevated temperature (ETD) was carried out at various temperatures within the range of 200 F. to the lower critical temperature for the steel composition. The data for the annealed and elevated temperature reduction step was selected as the most desirable at the various temperatures employed in the range of temperatures employed in the elevated temperature reduction step.

The data set forth in the following tables is not intended to provide comparisons for distinguishing the instant invention over values secured by steels processed in the manner described in the aforementioned copending applications, although some of the data will indicate the presence of improvements in some of the properties of the steels processed in accordance with the practice of this invention. It will be understood that the combination of steps which combines annealing in advance of elevated temperature reduction permits the development of properties in steel which heretofore have been incapable of formation, in many instances, in the steel by conventional methods of cold finishing, or even by previously described methods of drawing at elevated temperature.

Table I.C1144 steel air cooled after drawing Total Elon- Red. Izod Hardred. Tensile Yield gation of Warpage factor imness, Process porstrength, strength, 1.4, area, range pact, DPN,

cent 11.5.1. p.s.i. perper- F., MR

cent cent tin-lbs.

Annealed hot roll 86, 500 55, 000 28.0 40. 6 054 (47. 4) 167 Annealed (201d drawn(A) 21. 6 112, 500 97, 500 10. 0 36. 2 749 ll. 3 246 Ann.+ETD(A) 21.6 e 129, 500 B 126, 000 b 19. 5 b 44. 4 624 to 057 b 6.0 276 Ann. temp. 1,450 F. n 650 F. b 0 F. Number in parentheses denotes averaged value for fibrous fracture-not a clean break test.

Table lI.C-1144 steel quenched in oil or water after drawing Total Elon- Red. Izod Hardred., Tensile Yield gation of Warpcge factor imness, Process perstrength, strength, 14', area, range pact, DPN,

cent p.s.l. p.s.1. perper- 70 F., MR

cent cent ft.-lbs.

Annealed hot roll 86, 500 55, 000 28.0 40. 6 052 (47. 7) 167 Annealed cold drawn(0)..- 21.6 111.750 98,000 10.0 33.9 770 17.0 258 Ann.+ETD(O) 21. 6 e 128, 750 B 125, 500 d 16.0 d 33.9 620 to +.149 17. 7 271 Ann. temp. 1,450 F. 650

' 960 Number in parentheses denotes averaged value for fibrous fracture-not a. clean break test.

Table HIP-C1080 steel air cooled after drawing Total Elon- Red. Izod Hardred., Tensile Yield gatlon of Warpage factor imness, Process perstrength, strength, 1.4", area, range pact, DPN,

cent p.s.l. p.s.l. perper- 70 F., MR

cent cent it.-1bs.

Annealed hot roll 98, 000 43, 000 20.0 28. 4 043 8. 181 Annealed cold drawn (A)- 15.7 109, 500 100,500 8.0 24.5 109 3.7 258 Ann.+ETD (A) 15. 7 129, 000 127, 000 l 15. 0 1 31. 2 085 to 023 f 5. 3 280 Ann. temp.-1.450 F. e -at 020 F. I --at 910 F.

Table I V.C-1 080 steel quenched in oil or water Total Elon- Red. Izod Hardred., Tensile Yield gation of Warpage factor imness, Process perstrength, strength, 1.4, area, range pact, PN,

cent p.s.i. p.s.i. perper- 70 F., MR

cent cent it.-1bs.

Annealed hot roll 98, 000 43, 000 20.0 28. 4 043 8. 0 181 Annealed cold drawn (0). 15. 7 115,000 100,000 9. 5 26.0 132 4.0 266 Ann.+ETD (0) 15. 7 129, 000 125, 000 h 15. 0 h 33. 9 101 to +.008 7.3 276 Ann. temp.at 1,450 F. c --at 640 F. b --at 025 F.

Table V.4140 stel air cooled after drawing Total Elon- Red. Izod Hardred., Tensile Yield gation of Warpage factor lmness, Process perstrength, strength, 1.4, area, range pact, DPN,

cent p.s.i. p.s.i. perper- 70 F., MR

cent cent lt.-lbs.

Annealed hot roll 95, 500 53,000 28. 0 50.1 029 (79. 3) 184 Annealed cold drawn (A). 19.9 110, 500 105,000 16.5 54.8 013 (60.0) 249 Ann.+ETD (A) 19. 9 I 120, 000 117, 000 1 20.0 I 55.9 +.019 to 083 l 32. 271

Annealed-at 1,500 F. 1 at 780 F. i at 960 F. Numbers in parentheses denote averaged value for fibrous fracture-not a clean break test.

Table VI.-4140 steel quenched in oil after drawing Total Elon- Red. Izod Hardred., Tensile Yield gation of Warpage factor lmness, Process perstrength, strength, 1.4", area, range pact, DPN,

cent p.s.i. p.s.l. perper- 70 F., MR

cent cent it.-lbs.

Annealed hot roll 95, 500 53,000 28. 0 50. 1 029 (79. 3) 184 Annealed cold drawn (0). 19.9 111,500 106, 250 14.5 54.4 +.038 (62. 3) 249 Ann.-|-ETD (O) 19.9 is 124.000 124,000 I 22.0 55.9 +.057 to +.032 40.5 285 Annealed-at 1,500 F. k at 770 F.

Numbers in parentheses denote averaged values for fibrous fracture-not a clean break test.

It will be seen from a comparison of the data in Tables I and VI that when the steel is annealed and drawn to effect reduction at elevated temperature, important improvements are secured in most all of the properties when compared to the values secured when the same steel is annealed and drawn at room temperature. The same relationship and improvement are made available when the steels are quenched in oil or water after drawing or when they are allowed to cool down slowly in air after drawing. The advantages of quenching after drawing as distinguished from air cooling appear to lie principally in improved impact strength when the steel is drawn at a temperature above 700 F. Within the range, compressive stress values are also secured in the surface portion of the steels which are rapidly cooled as by quenching in oil or water immediately after drawing at elevated temperature.

The following tables indicate the changes which take place in the values secured for the various steels in response to the change in temperature of the steel advanced through the draw die to efiect reduction in cross-sectional area after the steels have been given a full annealing by previous heat treatment. The data set forth thus illustrates the improvement in the physical and mechanical properties of the steel by drawing the annealed steel while the steel is at an elevated temperature as compared to the values secured by drawing the same steel annealed in the same way to eifect an equivalent reduction but at room temperature. The temperature employed in the development of the data is illustrative of the step of drawing the annealed steel through a die to effect reduction in cross-sectional area while the steel is at an elevated temperature within the range of 200 F. but lower than the critical temperature for the steel composition (1200-1300 F.) and at a temperature within the preferred range of 400-900 F.

Table VII.--C1 144 steel drawn to 21.6 percent reduction [Air cooled after drawing, annealed 1.450 F.]

Table XI.-4140 steel drawn. to 19.9 percent reduction [Air cooledafter drawing, annealed 1,500 F.]

v Elon- Red War- Izod Hard- Elon- Red. W Izod Hard- Temp. of Tensile Yield gation, of page imp ct, 11 5 Temp. of Tensile Yield gation, of :2 impact, ness, draw, strength, strength, perarea, factor 70 F., DPN, draw, strength, strength, perarea, 70 F., DPN, n- -L psi. ent per- (cqnv. f -l MR F. p.s.i. p.s.l. cent, per- M ft.-lbs. MB

cent die) cent Hot roll 108.000 70, 500 23.0 46.1 +.004 32.7 220 Hon- 111 0,000 105 750 41 ,004 g 307 Annealed 1 Annealed 9 1 Not drawn-as received.

5 Not drawn.

Number in parentheses denotes averaged value for a fibrous fracture-- not a clean break test.

Table VIIl.C-1144 steel drawn to 21.6 percent reduction [Quenched in oil alter drawing, annealed 1,450 F.]

Elon- Red. War- Izod Eard- Temp. oi Tensile Yield gation, of page impact, ness,

draw, strength, strength, perarea, factor 70 DPN, F. p.s.i. p.s.i. cent per- (conv. ft.-lbs. MR

cent die) Hotroll 108,000 70,500 23.0 46.1 +.004 32.7 220 Annealed I Not drawn-as received.

9 Not drawn.

Number in parentheses denotes averaged values for a fibrous raeturenot a clean break test.

Table IX .-C1 080 steel drawn to 15.7 percent reduction [Air cooled after drawing, annealed 1,450 F.]

, Elon- Red. Wan Izod Hard- Temp. of Tensile Yield gation, of page impact, ness,

draw, strength, strength, perarea, factor F., DPN, F. p.s.i. p.s.1. cent perft.-lbs. MR

cent

I Not drawn-as received. 2 Not drawn.

Table X.--C1080 steel drawn to 15.7 percent reduction [Quenched in 011 after drawing, annealed 1,450 F.]

1 Y M Elon- Reid. Izodt Hard- Temp. of Tensl e le gation, o impac ness,

draw, strength, strength, perarea, 355 70 F., DPN, F. p.s.i. p.s.i. cent perft.-lbs. MR

cent

Hot roll L- 144, 500 76, 000 12. 5 17. 0 025 4. 7 296 Annealed 1 Not drawn-as received. 4 Not drawn.

1 Not drawn-as received.

I N 01; drawn.

Numbers in parentheses denote averaged values for fibrous fracturenot a clean break test.

Table XI1.-4-140 steel drawn to 19.9 percent reduction [Quenched in oil alter drawing, annealed 1,500 F.]

Elon- Red. Izod Hard- Temp. of Tensile Yield gation, of a e, impact, ness, draw, strength, strength, perarea, g g 70 F., DPN, F. p.s.i. p.s.i. cent perft.-lbs. MR

cent

1 Not drawn-us received.

Not drawn.

Numbers in parentheses denote averaged values for fibrous fracturenot a clean break test.

It will be apparent from the foregoing that desirable new and improved characteristics can be developed in steel by the combination of annealing the steel prior to reduction while the steel is at an elevated temperature within the range described. Desirable new and improved properties are secured, especially from the standpoint of ductility, residual stress and pull load, in addition to the developments of such new combinations of properties at high strength levels. While the characteristics of the types described are capable of development when the reduction step is carried out while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, it will be apparent that the improvements are maximized, especially from the standpoint of impact strength and other strength properties, when the steel is reduced while at a temperature within the range of 400-900 F.

From the foregoing it will be apparent that, by the combination of steps comprising annealing and reduction at elevated temperature, the strength properties for elevated temperature reduction can be maintained while concurrently improving the ductility of the steels. These improvements are secured in addition to improved machinability by comparison with steel reduced at elevated temperature without first annealing, and these improvements are secured in addition to greater uniformity of properties from bar to bar between heats by comparison with cold drawing of hot rolled steels and even drawing hot rolled steels at elevated temperature.

It will be understood that various changes may be made in the details of processing of the steel without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. The metallurgical process for the improvement of physical and mechanical properties of steel of the non- 11 austenitic type having a pearlitic structure in a matrix of free ferrite, comprising the combination of steps which includes advancing the steel through a draw die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, and annealing the steel prior to advancement of the steel through the draw die for the elevated-temperature-reduction step.

2. The metallurgical process for the improvement of physical and mechanical properties of steel of the nonaustenitic type having a pearlitic structure in a matrix of free ferrite, comprising the combination of steps which includes advancing the steel through a draw die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 400900 F. to the lower critical temperature for the steel composition, and annealing the steel prior to advancement of the steel through the draw die for the elevated-temperature-reduction step.

3. The metallurgical process for the improvement of physical and mechanical properties of steel of the nonaustenitic type having a pearlitic structure in a matrix of free ferrite, comprising the combination of steps which includes advancing the steel through an extrusion die to efiect reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, and annealing the steel prior to advancement of the steel through the extrusion die for the elevated-temperature-reduction step.

4. The metallurgical process for the improvement of physical and mechanical properties of steel which strain hardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, comprising the combination of steps which includes annealing the steel and subsequently working the annealed steel to effect reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

5. The metallurgical process for the improvement of physical and mechanical properties of steel which strainhardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, comprising the combination of steps which includes annealing the steel and subsequently advancing the steel through a draw die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

6. The metallurgical process for the improvement of physical and mechanical properties of steel which strainhardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, comprising the combination of steps which includes annealing the steel and subsequently advancing the steel through an extrusion die to etfect reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

7. The metallurgical process for the improvement of physical and mechanical properties of steel which strainhardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, comprising the combination of steps which includes annealing the steel and subsequently advancing the annealed steel through a die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, and air-cooling the steel after advancement through the die at elevated temperature.

8. The metallurgical process for the improvement of physical and mechanical properties of steel which strainhardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, comprising the combination of steps which includes annealing the steel and subsequently advancing the annealed steel through a die to effect reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, and quenching the steel rapidly to cool the steel after advancement through the die at elevated temperature.

9. The metallurgical process for the improvement of physical and mechanical properties of steel which strainhardens and hardens by some mode of precipitation when worked at a temperature between 200 F. and the lower critical temperature for the steel composition, comprising the combination of steps of annealing the steel, and subsequently rolling the annealed steel to effect reduction in cross-sectional area in a rolling operation while the steel is at a temperature within the range of 400-900 F., and then cooling the steel to ambient temperature after emission from the draw die at elevated temperature.

10. A steel product having improved physical and mechanical properties produced by the method of claim 4.

References Cited in the file of this patent UNITED STATES PATENTS Potter Feb. 20, 1912 Fawcett et a1. Mar. 1, 1949 OTHER REFERENCES 

1. THE METALLURGICAL PROCESS FOR THE IMPROVEMENT OF PHYSICAL AND MECHANICAL PROPERTIES OF STEEL OF THE NONASUSTENITIC TYPE HAVING A PEARLITIC STRUCTURE IN A MATRIX OF FREE FERRITE, COMPRISING THE COMBINATION OF STEPS WHICH INCLUDES ADVANCING THE STEEL THROUGH A DRAW DIE TO EFFECT REDUCTION IN CROSS-SECTIONAL AREA WHILE THE STEEL IS AT A TEMPERATURE WITHIN THE RANGE OF 200* F. TO THE LOWER CRITICAL TEMPERATURE FOR THE STEEL COMPOSITION, AND ANNEALING THE STEEL PRIOR TO ADVANCEMENT OF THE STEEL THROUGH THE DRAW DIE FOR THE ELEVATED-TEMPERATURE-REDUCTION STEP. 